1. Field of the Invention
This invention relates to semiconductor devices and more particularly to semiconductor devices having plated contacts, and methods of manufacturing the same.
2. Description of Related Art
Extending the bandwidths of semiconductor devices, such as heterojunction bipolar transistors (HBTs), often requires submicron scaling of the lateral or width dimensions of a semiconductor junction and close positioning, relative to each other, of the electrical contacts to the semiconductor layers that form the junction. Presently, submicron scaling of emitter-base junctions in HBTs is difficult because of the process steps used to fabricate the device. “Submicron” as used herein means less than or equal to approximately 1 micron (μm).
Two process steps make the formation of a submicron base-emitter junction difficult. They are: 1) the formation of a submicron emitter contact having a high aspect ratio and substantially straight sidewalls relative to the semiconductor surfaces, and 2) the deposition of a base contact that is in close proximity to the emitter contact. Because of alignment tolerances in lithographic processes, self-aligned processes are preferred for depositing contacts in close proximity to one another. “Self-aligned processes” as used herein means processes that rely on only one lithography operation to set the position of one feature relative to another. “Aspect ratio” as used herein means the ratio of emitter contact height to contact width.
A conventional approach for forming an emitter contact involves electron beam evaporation and liftoff processes. As illustrated in FIGS. 1a and 1b, these processes create an emitter contact 100 with a tapered profile. A standard evaporation process creates an angle of approximately 75 degrees between the sidewall of the contact and the semiconductor surface. This limits the maximum aspect ratio of the contact to approximately 1.75. The tapered profile also provides less area for interconnect metal to contact the emitter.
Another approach to forming the emitter contact, as illustrated in FIGS. 2a and 2b is to blanket deposit the emitter metal 102 on the emitter layer 104 of semiconductor material (FIG. 2a). The emitter metal 102 is then patterned with a masking material 106 and etched to leave an emitter contact 108 (FIG. 2b). The etching leaves the emitter contact 108 under the masking material 106. Metal deposition (FIG. 2a) and etch processes (FIG. 2b) suffer from problems such as: metal film stress, etch mask selectivity and etch undercut.
During formation of an HBT, the emitter contact is typically used as an etch mask for a self-aligned etch of the emitter mesa. In this process (FIG. 1b, FIG. 2c), the emitter semiconductor layer 104 is etched away, leaving the emitter contact 100, 108 and the underlying, remaining portion 110 of the emitter layer. Thus, the contact 100, 108 must be scaled to approximately the same dimensions as the emitter-base junction. As illustrated in FIG. 1b and FIG. 2c, the etching may result in a contact 100, 108 that is undercut which, as explained below, impacts device yield.
In one standard fabrication technique, the emitter-contact must be tall (approximately 1 μm) to allow process margin in the planarization and etch back process used to contact the HBT terminals with the first level of metal interconnect. Therefore, submicron devices require emitter contacts with large aspect ratios. Such dimensions are difficult to realize using standard evaporation and liftoff processes because of the tapered profile (FIGS. 1a and 1b) that is obtained.
To minimize base resistance and base-collector capacitance in the device, the base contact should be placed as close as possible to the emitter contact. In most instances, as the device is scaled down, the base contact to emitter contact separation must also decrease. A non-self-aligned deposition of the base contact is difficult to realize for base-to-emitter contact separations of less than 0.5 μm. Self-aligned processes are preferred to deposit the base contact in close proximity to the emitter contact.
One standard approach to depositing a self-aligned base contact is to undercut the emitter semiconductor beneath the emitter contact using a wet chemical mesa etch to produce an undercut emitter contact 100, 108 as illustrated in FIG. 1b and FIG. 2c. As illustrated in FIGS. 3a-3b, the undercut emitter contact 108 is then used as a shadow mask during electron beam evaporation of the base contact 112. The line-of-sight nature of the evaporation process is supposed to prevent base-emitter short circuits. However, this process generally suffers from poor yield. The undercut of the emitter semiconductor 110 is difficult to control; as a result, the undercut emitter contact 108 may separate from the emitter semiconductor 110. Additionally, in order to avoid emitter-base short circuits 114 during deposition of the base contact, the thickness of the base contact must be less than the thickness of the emitter semiconductor 110. Even if this requirement is met, any stray metal strands inadvertently deposited during the liftoff process may cause a base-emitter short circuit.